COMP 421 /CMPET 401

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COMP 421 /CMPET 401

• COMMUNICATIONS and NETWORKING
• Chapter 3
• Data Transmission

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Review Connection/Connectionless
Service Example Reliable Message Stream
Sequence of Pages Reliable byte stream Remote
logon Unreliable connection Digitized
Voice Unreliable datagram Electronic Junk
Mail Acknowledged Datagram Registered
mail Request-reply Database Query

Connection- oriented

Connection- less
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Review Connection/Connectionless

• Connection-oriented service is modeled after the
  Telephone Company
• Connectionless Service is modeled after the
  Postal System

PRIMITIVE MEANING Request A Entity wants the
service to do something Indication A Entity is
informed about an event Response An Entity wants
to respond to an event Confirm The response to
an earlier request has come back
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A Sample Connection Oriented Service
CONNECT.request Request a connection CONNECT.indic
ation Signal the called Party CONNECT.response Cal
lee accepts or rejects call CONNECT.confirm Tell
Caller whether call was accepted DATA.request Req
uest that data be sent DATA.indication Signal
the arrival of data DATA.response Request that
connection be released DATA.confirm Signal peer
about request
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Layer N1
Computer 1
Layer N
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6

1 2 3 4 5 6 7 8 9 10
Time
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Layer N1
Computer 2
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8

2
Layer N
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LAST WEEK - OSI

• We Spoke about the OSI/ISO and TCP/IP Models
• NEITHER the OSI model and its Protocols nor
• the TCP/IP models and its protocols are perfect
• Bad Timing
• Bad Technology
• Bad Implementations
• Bad Politics.

• OSI Model is
• Printed Standards almost a meter thick
• The standards are difficult to implement
• The stands are inefficient in operation

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LAST WEEK - TCP/IP

• The TCP/IP Model is
• The first implementation of TCP/IP was part
• of Berkeley UNIX and was good
• The model does not clearly distinguish the
  concept of
• Service
• Interface
• Protocol
• The TCP/IP model is NOT general and is poorly
• suited for describing any protocol other than
  TCP/IP
• The TCP/IP model does not distinguish between the
  
• Physical and Data Link Layers, which are
  completely different
• While the TCP and IP stack are well thought out
  and
• implemented, many of the other protocols were Ad
  Hoc, generally
• produced by a couple of Grad Students hacking
  away until they got tired

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DECIBELS

• Decibels are often used in communications when
• Talking about signal strength
• Talking about the net gain or loss of a cascaded
  transmission path
• A Decibel is a measure of the ratio between two
  signal levels
• N 10logP2/P1 N number of decibels
• P1input power level
• P2output power level
• dBW (decibel-watt) is the absolute power level
• Power 10log Power (watts)/1(watt)
• 1mW -30dBW
• 1 W 0 dBW
• 1000W 30dBW

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This Week The Physical Layer
Communications and Information Theory are topics
of whole courses Well cover some theoretical
basics regarding communications over a physical
channel We discover that there are physical
limitations to communications over a given
channel Well cover some fundamental theorems
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Physical Layer
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Physical / Data Link Layer Interface
Sender
Receiver
NL
HDR
DLL
Frame
ACK
PL
HDR
Transmitted Bits
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Transmission Terminology (1)

• Transmitter
• Receiver
• Medium
• Guided medium
• e.g. twisted pair, optical fiber
• Unguided medium
• e.g. air, water, vacuum

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Transmission Terminology (2)

• Direct link
• No intermediate devices
• Point-to-point
• Direct link
• Only 2 devices share link
• Multi-point
• More than two devices share the link

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Transmission Terminology (3)

• Simplex
• One direction (but in Europe means half duplex)
• e.g. Television
• Half duplex
• Either direction, but only one way at a time
• e.g. police radio
• Full duplex
• Both directions at the same time
• e.g. telephone

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Frequency, Spectrum, and Bandwidth

• Electromagnetic signal are used to transmit data
• This transmitted signal is a function of Time
• Time-Domain
• This transmitted signal can also be a function of
  Frequency
• Frequency-Domain
• The Frequency domain is more important in
  understanding
• data transmission

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Electromagnetic Signals

• Function of time
• Analog (varies smoothly over time)
• Digital (constant level over time, followed by a
  change to another level)
• Function of frequency
• Spectrum (range of frequencies)
• Bandwidth (width of the spectrum)

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Time domain concepts

• A Continuous signal
• Varies in a smooth way over time
• A Discrete signal
• Maintains a constant level then changes to
  another constant level
• A Periodic signal
• Pattern repeated over time
• An Aperiodic signal
• Pattern not repeated over time

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Periodic Signal Characteristics

• Amplitude (A) signal value, measured in volts
• Frequency (f ) repetition rate, cycles per
  second or Hertz
• Period (T) amount of time it takes for one
  repetition, T1/f
• Phase (F) relative position in time, measured in
  degrees or radians

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Analog Signaling

• represented by sine waves

1 cycle
amplitude (volts)
phase difference
time
(sec)
frequency (hertz)
cycles per second
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Digital Signaling

• represented by square waves or pulses

1 cycle
amplitude (volts)
time
(sec)
frequency (hertz)
cycles per second
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BPS vs. Baud

• BPSbits per second
• Baud of signal changes per second
• Each signal change can represent more than one
  bit, through variations on amplitude, frequency,
  and/or phase

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Continuous Discrete Signals
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PeriodicSignals
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Sine Wave

• Peak Amplitude (A)
• maximum strength of signal
• volts
• Frequency (f)
• Rate of change of signal
• Hertz (Hz) or cycles per second
• Period time for one repetition (T)
• T 1/f
• Phase (?)
• Relative position in time

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Varying Sine Waves
Sin2pt
0.5Sin2pt
Phase Shift in radians
or
Sin4pt
Phase Shift in seconds
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Wavelength (?)

• Distance occupied by one cycle
• Distance between two points of corresponding
  phase in two consecutive cycles
• Assuming signal velocity in space is equal to v
• ? vT or
• ?f v
• Here, v c 3108 ms-1 (speed of light in free
  space)
• Remember T1/ f

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Frequency Domain Concepts

• A Signal is usually made up of many frequencies
• Components are sine waves
• It Can be shown (Fourier analysis) that any
  signal is made up of component sine waves
• One can plot frequency domain functions instead
  of/in addition to time domain functions

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Addition of FrequencyComponents
(a) Sin(2pft)
(b) (1/3)Sin(2p(3f)t)
(c) (4/p)Sin(2pft)(1/3)Sin(2p(3f)t)
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Communications Basics

• Represent a signal as a single-valued function of
  time, g(t), to model behavior of a signal (may be
  voltage, current or other change)
• Jean-Baptiste Fourier showed we can represent a
  periodic signal (given some conditions) as the
  sum of a possibly infinite number of sines and
  cosines

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Original
Harmonic spectrum
As we add more harmonics the signal reproduces
the original more closely
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Signal Transmission

• No transmission facility can transmit signals
  without losing some power
• Usually this attenuation is frequency dependent
  so the signal becomes distorted
• Generally signal is completely attenuated above
  some max frequency (due to medium characteristics
  or intentional filtering)
• The signal is bandwidth limited

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Signal Transmission

• Time T necessary to transmit a character depends
  on coding method and signaling speed
• Signaling speed number of times per second the
  signal changes value and is measured in baud
• Note that baud rate is not necessarily the same
  as the bit rate
• By limiting the bandwidth of the signal we also
  limit the data rate even if a channel is perfect
• Overcome this by encoding schemes

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Spectrum Bandwidth

• Spectrum
• range of frequencies contained in signal
• Absolute bandwidth
• width of spectrum
• Effective bandwidth
• Often just bandwidth
• Narrow band of frequencies containing most of the
  energy
• DC Component
• Component of zero frequency

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Signal with DC Component
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Data Rate and Bandwidth

• Any transmission system has a limited band of
  frequencies
• This in turn limits the data rate that can be
  carried

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Bandwidth

• Width of the spectrum of frequencies that can be
  transmitted
• if spectrum300 to 3400Hz, bandwidth3100Hz
• Greater bandwidth leads to greater costs
• Limited bandwidth leads to distortion
• Analog measured in Hertz
• Digital measured in baud or Bps

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Analog and Digital Data Transmission

• Data
• Entities that convey meaning
• Signals
• Electric or electromagnetic representations of
  data
• Transmission
• Communication of data by propagation and
  processing of signals

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Voice Grade Line

• For a given Bit Rate of b bits/sec the time
  required to send 8 bits is b/8 Hz.
• For a voice Grade Line has a cutoff frequency
  near 3000Hz
• This restriction means that the number of the
  highest harmonic passed through is 3000/(b/8) or
  24000/b

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Data

• Analog
• Continuous values within some interval
• e.g. sound, video
• Digital
• Discrete values
• e.g. text, integers

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Acoustic Spectrum (Analog)
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Signals

• Means by which data are propagated
• Analog
• Continuously variable
• Various media
• wire, fiber optic, space
• Speech bandwidth 100Hz to 7kHz
• Telephone bandwidth 300Hz to 3400Hz
• Video bandwidth 4MHz
• Digital
• Use two DC components

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Digital Text Signaling

• Transmission of electronic pulses representing
  the binary digits 1 and 0
• How do we represent letters, numbers, characters
  in binary form?
• Earliest example Morse code (dots and dashes)
• Most common current form ASCII

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ASCII Character Codes

• Use 8 bits of data (1 byte) to transmit one
  character
• 8 binary bits has 256 possible outcomes (0 to
  255)
• Represents alphanumeric characters, as well as
  special characters

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Digital Image Signaling

• Pixelization and binary representation

Code 00000000 00111100 01110110 01111110 011
11000 01111110 00111100 00000000
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Bit rate and Baud rate

• Bit rate number of bits that are transmitted in
  a second
• Baud rate number of line signal changes
  (variations) per second
• If a modem transmits 1 bit for every signal
  change
• bit rate baud rate
• If a signal change represents 2 or more or n bits
• bit rate baud rate n

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Data and Signals

• Usually use digital signals for digital data and
  analog signals for analog data
• Can use analog signal to carry digital data
• Modem
• Can use digital signal to carry analog data
• Compact Disc audio

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Why Study Analog?

• Telephone system is primarily analog rather than
  digital (designed to carry voice signals)
• Low-cost, transmission medium (present almost at
  all places at all times
• If we can convert digital information (1s and 0s)
  to analog form (audible tone), it can be
  transmitted inexpensively

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Voice Signals

• Easily converted from sound frequencies (measured
  in loudness/db) to electromagnetic frequencies,
  measured in voltage
• Human voice has frequency components ranging from
  20Hz to 20kHz
• For practical purposes, the telephone system has
  a narrower bandwidth than human voice, from 300
  to 3400Hz

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Analog Signals Carrying Analog and Digital Data
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QAM

• QAM - Quadrature Amplitude Modulation

• Diagrams that show legal combinations of
  amplitude and phase
• are called CONSTELLATION PATTERNS

2 bits/Baud 8 Valid combinations 4800bps
4 bits/Baud 16 valid combinations 9600bps ITU
V.32 modem standard

• The next step after 9600bps is 14400bps and is
  called V.32 bis (transmits 6 bits)
• This is followed by V.34 running at 28,800bps
  with 128 bit constellation

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Digital Signals Carrying Analog and Digital Data
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Analog Transmission

• Analog signal transmitted without regard to
  content
• May be analog or digital data
• Attenuated over distance
• Use amplifiers to boost signal
• Also amplifies noise

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Digital Transmission

• Concerned with content
• Integrity endangered by noise, attenuation etc.
• Repeaters used
• Repeater receives signal
• Extracts bit pattern
• Retransmits
• Attenuation is overcome
• Noise is not amplified

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Advantages of Digital Transmission

• Digital technology
• Low cost LSI/VLSI technology
• Data integrity
• Longer distances over lower quality lines
• Capacity utilization
• Economical high bandwidth links
• High degree of multiplexing easier with digital
  techniques
• Security Privacy
• Encryption
• Integration
• Can treat analog and digital data similarly

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Transmission Media

• The physical path between transmitter and
  receiver is the Transmission Path
• Design factors
• bandwidth
• attenuation weakening of signal over distances
• interference
• number of receivers

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Impairments and Capacity

• Impairments exist in all forms of data
  transmission
• Analog signal impairments result in random
  modifications that impair signal quality
• Digital signal impairments result in bit errors
  (1s and 0s transposed)

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Transmission Impairments

• Signal received may differ from signal
  transmitted
• Analog - degradation of signal quality
• Digital - bit errors
• Caused by
• Attenuation and attenuation distortion
• Delay distortion
• Noise

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Transmission Impairments

• Attenuation
• loss of signal strength over distance
• Attenuation Distortion
• different losses at different frequencies
• Delay Distortion
• different speeds for different frequencies
• Noise

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Attenuation
P2 watts
P1 watts
receiver
transmitter
10 log10 (P1/P2) dB
Attenuation
10 log10 (P2/P1) dB
Amplification
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Attenuation

• Signal strength falls off with distance
• Depends on medium
• Received signal strength
• must be enough to be detected
• must be sufficiently higher than noise to be
  received without error
• Attenuation is an increasing function of
  frequency

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Delay Distortion

• Occurs only in guided media
• The velocity of propagation of a signal through a
  guided medium varies with frequency.
• This effect is called delay distortion
• Its affect is the received signal is distorted
  due to varying delays
• Its more critical in digital data
• Because of delay distortion some of the signal
  components in one bit position can spill into
  another causing intersymbol interference which is
  a major limitation to the maximum bit rate in a
  transmission channel

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Noise (1)

• Noise is the major limiting factor in
  communication system performance
• Noise is the unwanted signals that inserted
  between transmitter and receiver

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Noise (2)

• There are 4 main types of Noise
• Thermal
• Due to thermal excitement of electrons
• Uniformly distributed, cannot be eliminated
• Noise is assumed to be independent of frequency
• White noise
• Intermodulation
• Signals that are the sum and difference of
  original frequencies sharing a medium

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Noise (3)

• Crosstalk
• A signal from one line is picked up by another
• NEXT (near-end crosstalk )
• interference in a wire at the transmitting end of
  a signal
• sent on a different wire
• FEXT (far-end crosstalk)
• interference in a wire at the receiving end of a
  signal
• sent on a different wire
• Impulse
• Irregular pulses or spikes
• e.g. External electromagnetic interference
• Short duration
• High amplitude
• Less predictable

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Noise (4)

• Effect of Noise is
• distorts a transmitted signal
• attenuates a transmitted signal
• The signal-to-noise ratio to quantifies noise by
  expressing in decibels the amount by which a
  signal level exceeds the noise within a specific
  bandwidth

S N
S/Ndb
10 log
N noise power
S average signal power
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Effect of noise
Logic Threshold
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Channel Capacity

• Data rate
• In bits per second
• Rate at which data can be communicated
• Bandwidth
• In cycles per second of Hertz
• Constrained by transmitter and medium

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Maximum Data Rate

• In 1920s Nyquist (of the Nyquist Theorem)
  developed an equation for the maximum data rate
  of a noiseless channel
• For low pass filtered signal of bandwidth B
• Sampling at exactly 2B samples per sec allows
  reconstruction of the signal
• More samples are useless since the frequencies
  above B are filtered out

CCapacitymax data rate 2B log2 M bits/sec for
M discrete levels
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Nyquist theorem

• In a perfectly noiseless channel, if f is
  the maximum frequency the medium can transmit,
  the receiver can completely reconstruct a signal
  by sampling it 2f times per second
• Nyquist, 1920

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Nyquist formula
B bandwidth M number of discrete signal levels
2B log2 M
C
Theoretical capacity for Noiseless channel
Example Channel capacity calculation for voice
bandwidth (3100 Hz)

• M Max data rate (C)
• 2 6200 bps
• 4 12400 bps
• 8 18600 bps
• 16 24800 bps

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Shannons Law

• In the 40s Shannon (of Shannons Law) extended
  the equation to a channel subject to
  thermodynamic (thermal) noise
• Thermal noise measured by ratio of signal (S)
  power to noise (N) power (signal-to-noise ratio -
  S/N)
• But represented as 10 log10 S/N
• These units are called decibels (dB)
• Now, for a channel with signal to noise of S/N

CapacityCmax bits/sec B log2 (1 S/N)
Here, CTheoretical Maximum capacity with noise
Note Only much lower rates are achieved since
the equation assumes zero impulse noise and no
attenuation and delay distortion.
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Maximum Data Rate of a Noisy Channel
For a channel of 30,000Hz bandwidth and a signal
to thermal noise ratio of 30dB The best that
can be transmitted is a little over 30,000bps No
matter how many or how few signal levels are
used and no matter how often or how infrequent
samples are taken
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The Telephone Company
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The Telephone Network

• The telephone network consists of your phone at
  home that is connected (by the Local Loop) to the
  Central Office. The Central Office is in turn
  connected to a Hierarchical Phone Network.
  Worldwide, there are over 300 million
  (300,000,000) telephones - 98 of them
  interconnected.
• POTS - Plain Old Telephone Set
• The POTS, or Plain Old Telephone Set, consists of
  these 5 sections
• Ringer Unit
• Hook Switch
• Dialer Unit
• Hybrid/Speech Network
• Hand Set

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POTS
The connection to the CO (Central Office)
comprises only 2 wires Tip and Ring. This
connection is called the "Local Loop."
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The Local Loop
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Tip Ring
The Tip is ve and colored green. The Ring is -ve
and colored Red. If you look at a phone jack in
your house, you will see that it is wired for 4
wires Red, Green, Black and Yellow. However,
black and yellow are not normally used. The
black and yellow wires can be used for a second
telephone line or they can be used for running a
Network Physical layer protocol called Phonenet
(by Farralon). Phonenet uses the black and yellow
for Network communications. It is for use with
Appletalk, and is a replacement for Localtalk. It
runs at the Localtalk speed of 230 Kbps,
reasonable for small networks.
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Ringer Unit

• Ringer Unit
• The ringer is a device that alerts you to an
  incoming call it interprets the ringing voltage
  from the Central Office. Originally, the ringer
  was a electromagnetic bell. Today, though, most
  ringers are electronic devices.
• The Central Office sends the following
• a 90 to 120 VAC ringing voltage
• Frequency of 20 Hz
• Cadence for North America is 2 sec On/ 4 sec Off

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The Hook Switch
Hook Switch The hook switch is activated by
lifting the handset off of the cradle. The
position of the hook switch determines whether
the telephone is waiting for a call, or is
actively using the line. The off-hook position
informs the network of a request for use. The
on-hook position releases the use of the network.

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The Dialer Unit
Dialer Unit There are two types of Dialer Units
Rotary and Touch Tone. Rotary is the old "put
your finger in the hole and spin" type. The
rotary dial operates by toggling the Hook Switch
on and off.
Touch Tone is the modern method where 2
frequencies per push button are sent. Touch Tone
is a trade name the correct name is DTMF (Dual
Tone Multi Frequency).
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Hybrid/Speech Network

• Hybrid/Speech Network
• The Hybrid/Speech Network performs these
  functions
• It converts the Tx/Rx 4 wires from the Handset to
  the 2 wires for the Local Loop.
• It interfaces the signals from the Dialer Unit to
  the telephone line.
• It provides auto line compensation for line
  length to keep the volume constant.

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The Handset

• Handset
• The Handset contains transducers that convert
  mechanical energy into electrical energy. The
  microphone converts speech into electrical energy
  while the diaphragm (or speaker) converts
  electrical signals into audible signals.
• Functions of a Telephone Set are shown below.
• Request use of network from the CO (Central
  Office).
• Inform you of the network status Dial-tone,
  Ringing, Busy, Fast Busy (Talk Mail)
• Informs CO of desired number.
• Informs you when a call is incoming (phone
  rings).
• Releases use of network when call is complete
  (hang-up)
• Transmit speech on network receives speech from
  distant caller.
• Adjust power levels and compensates for line
  length

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Local Loops
Local Loops The Local Loop is the connection
between the Central Office and the home or
business. Two wires (1 pair) are run into every
home. The pair does not go directly to the
Central Office. Instead, it goes to those big
green boxes--that you see on the street
corners--called "Serving Area Interfaces" (SIA) .
Large multi-conductor bundles of wires then go
from there to the Central Office.
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TELCO Architecture
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The Central Office
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The Central Office (2)

• The Central Office provides the following
  functions
• It supplies the battery voltage for the telephone
  system. The On-hook voltage is 48 Vdc /- 2V.
  Off-hook voltage is -6.5 Vdc.
• It supplies the Ringing Generator - 90 to 120
  VAC, 20 Hz, 2 sec on/ 4 sec off
• It supplies the Busy signal (480 620 Hz, 0.5
  sec On/ 0.5 sec Off), Dial Tone (350 440 Hz)
  and Fast Busy (480 620 Hz, 0.2 sec On/ 0.3 sec
  Off).
• It has the digital switching gear that determines
  if the number is an Interoffice call (local) or
  an Intraoffice call (Toll - long distance).

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Central Office (3)
A Central Office can have up to 10,000
subscribers (for example, 284-0000 to 284-9999).
Most have 4,000 to 5,000 subscribers. The Central
Office bases the loading requirements on roughly
10 of the phones that will be in use at any one
time. However, the use of Internet dialup access
has drastically changed this statistic
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Hierarchical Phone Networks
The PSTN (Public Switch Telephone Network) is
divided into a hierarchical network. Here are the
5 classes of switching centers in North America
Center Class Description Abbreviation
Symbol
1 Regional Center RC
2 Sectional Center SC
3 Primary Center PC
4 Toll Center TC
4b Toll Point TP
5 Central Office CO
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An Example
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Hierarchical Structure
The Hierarchical portion is seen as follows
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Call Routing

• Call routing
• Preferred route
• Second choice
• Third Choice
• Call routing is determined by network engineering
  and physical location. When all lines are idle,
  the call routing selects the preferred route. If
  the preferred route is busy, then the call is
  routed to the second choice. Because the second
  choice is routed through one toll center, the
  charge for the call is greater than the preferred
  route. The third choice is used when the second
  choice is busy. The third choice goes through 2
  toll centers, and is the most expensive route

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END Class 3